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Kohaku

Slip casting globes- form?

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OffCenter    82

Hmmm

I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.

Spray with WD-40 and pour the plaster around the upper half.

I never had much success trying to hold something down in plaster.

 

Marcia

 

 

Damn! I wish I had thought of that! I can almost hear my dad saying, "There's the right way and there's Jim's way."

 

Jim

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Kohaku    22

Hmmm

I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.

Spray with WD-40 and pour the plaster around the upper half.

I never had much success trying to hold something down in plaster.

 

Marcia

 

 

Glad I saw this before forging ahead.

 

How about the second pour? Once you flipped and removed the sand/cardboard, how would you make sure the ball stayed put during pour #2? I was things of forming a pouring spout from clay and using this to hold the ball in place as I poured...

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Marcia Selsor    1,301

Hmmm

I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.

Spray with WD-40 and pour the plaster around the upper half.

I never had much success trying to hold something down in plaster.

 

Marcia

 

 

Glad I saw this before forging ahead.

 

How about the second pour? Once you flipped and removed the sand/cardboard, how would you make sure the ball stayed put during pour #2? I was things of forming a pouring spout from clay and using this to hold the ball in place as I poured...

 

that sounds feasible. make sure you carve the keys in to the first half before you pour the second half.Do you mean the slip reservoir or is this a pitcher.?

Marcia

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Kohaku    22

that sounds feasible. make sure you carve the keys in to the first half before you pour the second half.Do you mean the slip reservoir or is this a pitcher.?

Marcia

 

 

Sorry about the fuzzy terminology- slip reservoir is what I meant. My goal is to get as close to an immaculate, make-a-mathematician-weep sphere as I can get.

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DanMehlman    0

HI- I'm a professional model and mold maker in the ceramic and glass industry and I would like comment on your question and the responses so far. I've made such molds before.

 

1- You can't beat a bowling ball for that size sphere. Borrow one or get one at a thrift shop. They are so round that you can rotate the ball and cast both sections from the same side of the ball, thereby avoiding the finger holes. Billiard balls are excellent for their size. Bocce balls?

2- If you are trying to make a nice clean mold, don't try to push a rubber ball down into wet plaster, you will trap air and get voids on the casting surface. Always pour plaster on top of a model so air bubbles rise away from the casting surface.

3-  Any kind of inflated ball will give you problems because it will float and possibly distort. Even if you can secure it down with clay, tape, etc. for the first section, it will float when you pour the second section. The one possibility for section #2 is to create a structure to hold it down from overhead and bury that in the pour hole model. If the ball is soft, this will likely create distortion and may still float enough to come out of section #1 a bit, ruining the mold. If the ball is rigid, you may get away with this system.

4-  If I had to make a mold from an inflated ball I would do this: set the ball up in a clay pad up to the seam line. Splash on the first half of a plaster waste mold. Turn it over, carve keys, apply release to the seam line, splash on a second mold section. There will be no floating issues with a layed-up mold. Open the waste mold, remove ball, seal and soap the mold, reassemble, and fill with plaster. Break the waste mold away. You now have a plaster reproduction of the ball, which will not float when you make the working mold.

5- Personally, I would (and I have) make a plaster ball from scratch, either by turning two halves upright on the plaster wheel, or by turning it in one piece on the lathe... but this takes experience and equipment that you may not have.

6- Contrary to advice on this thread, do not use WD-40 or any oil as a mold release, it will seal the mold and impede the slip casting. Plastic or rubber balls need no release at all. Use mold soap as a release on the seam line when you make section #2.

 

Good luck with the project

Dan

www.MehlmanDesign.com

 

 

 

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Kohaku    22

HI- I'm a professional model and mold maker in the ceramic and glass industry and I would like comment on your question and the responses so far. I've made such molds before.

 

1- You can't beat a bowling ball for that size sphere. Borrow one or get one at a thrift shop. They are so round that you can rotate the ball and cast both sections from the same side of the ball, thereby avoiding the finger holes. Billiard balls are excellent for their size. Bocce balls?

2- If you are trying to make a nice clean mold, don't try to push a rubber ball down into wet plaster, you will trap air and get voids on the casting surface. Always pour plaster on top of a model so air bubbles rise away from the casting surface.

3-  Any kind of inflated ball will give you problems because it will float and possibly distort. Even if you can secure it down with clay, tape, etc. for the first section, it will float when you pour the second section. The one possibility for section #2 is to create a structure to hold it down from overhead and bury that in the pour hole model. If the ball is soft, this will likely create distortion and may still float enough to come out of section #1 a bit, ruining the mold. If the ball is rigid, you may get away with this system.

4-  If I had to make a mold from an inflated ball I would do this: set the ball up in a clay pad up to the seam line. Splash on the first half of a plaster waste mold. Turn it over, carve keys, apply release to the seam line, splash on a second mold section. There will be no floating issues with a layed-up mold. Open the waste mold, remove ball, seal and soap the mold, reassemble, and fill with plaster. Break the waste mold away. You now have a plaster reproduction of the ball, which will not float when you make the working mold.

5- Personally, I would (and I have) make a plaster ball from scratch, either by turning two halves upright on the plaster wheel, or by turning it in one piece on the lathe... but this takes experience and equipment that you may not have.

6- Contrary to advice on this thread, do not use WD-40 or any oil as a mold release, it will seal the mold and impede the slip casting. Plastic or rubber balls need no release at all. Use mold soap as a release on the seam line when you make section #2.

 

Good luck with the project

Dan

www.MehlmanDesign.com

 

Dan- that's really helpful- thanks.

 

I went on raku binge over the past couple weeks, and put this mold project on the back burner. I was gearing up to try with rubber balls though... and so appreciate the timely warning.

 

We do have a bocci set, actually. Unfortunately, the size I'm looking for is a bit bigger... so maybe a bowling ball is- after all- the way to go.

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In fact on my last visit to our local pottery supply, the proprietor and I were discussing slip casting (I was wondering about using paper forms dipped in slip -- I've since seen some work done like that seaching online, for example http://www.australianceramics.com/JUNE/origami.html/).

 

She told me that she once dipped a hollow plastic ball in slip, poked a small hole in it, and fired it and it came out well, with just a bit of ash or residue near the hole. Thought I'd pass that on.

 

[edit: oops, I see Marcia already said that this works in the post right before mine, tried to skim them all before posting but missed that, pfft]

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